FIG. 6 and FIG. 7 are a plan view and a side view, respectively, showing a head positioning apparatus of a conventional magnetic hard disk apparatus. A magnetic hard disk 102 is driven, through a spindle 104, by a spindle motor 106. A magnetic head 108 for writing data to the disk 102 and reading data from the disk 102 is mounted on one end of a carriage 110. The carriage 110 is fixed to a shaft 111, which is mounted, through a ball bearing 114, on a housing base 116 so that the carriage 110 can pivot on a center axis 112 of the shaft 111. The carriage 110 is fixed to the shaft 111 by using a general method in which a bolt is screwed into a tapped hole formed in the shaft 111. Fixed to an end opposite, with respect to the axis 112 of the shaft 111, to the one end of the carriage 110 on which the head 108 is mounted, is a coil 118. Above the coil 118, a permanent magnet 120 is provided at a predetermined distance from the coil 118 and, on the other hand, below the coil 118, a permanent magnet 122 is provided at a predetermined distance from the coil 118. The coil 118 and the permanent magnets 120 and 122 compose a voice coil motor. The carriage 110 is pivoted on the axis 112 by a force generated by a current flowing through the coil 118 and a magnetic field produced from the permanent magnets 120 and 122. A direction in which the carriage 110 pivots is determined by the current that flows through the coil 118.
FIG. 8 and FIG. 9 are a cross-sectional view and an exploded perspective view, respectively, showing a ball bearing assembly used for the conventional head positioning apparatus of FIG. 6 and FIG. 7. As shown in these figures, a ball bearing assembly 114 comprises an upper bearing section 114U and a lower bearing section 114D. For the upper bearing section 114U, grease 214U is applied to the top surface of a ball retainer 212U for retaining within a predetermined area, each of a plurality of balls 210U arranged between an inner race 206U formed integrally with the shaft 111 and an outer race 208U, and a shield ring 216U is provided on the top end of the outer race 208U to cover the grease 214U. For the lower bearing section 114D, grease 214D is applied to the bottom surface of a ball retainer 212D for retaining within a predetermined area, each of a plurality of balls 210D positioned between an inner race 206D formed integrally with the shaft 111 and an outer race 208D, and a shield ring 216D is provided on the bottom end of the outer race 208D to cover the grease 214D.
Also a ball bearing assembly disclosed in Japanese Published Unexamined Patent Application (PUPA) No. 1-316518 has a ball retainer with grease applied to its top surface.
In the conventional ball bearing assembly shown in FIG. 8 and FIG. 9, since remaining grease 214U flows into a gap between the inner race and shaft 111 and the retainer 212U as shown in FIG. 10, frictional force applied to the inner race and shaft 111 increases to prevent the shaft 111 from rotating smoothly. Also the grease 214D applied to the bottom surface of the retainer 212D of the lower bearing section 114D increases frictional force to the inner race and shaft 111.
Also in the case of the ball bearing disclosed in Japanese PUPA No. 1-316518, it is considered that remaining grease flows into a gap between an inner race and a bearing by which frictional force to the inner race is increased, as described above.
In a head positioning apparatus in which a carriage for supporting a head is mounted, through a bearing, on a housing base, the increase of frictional force to a shaft or an inner race caused by the remaining grease, as described above, causes the accuracy of positioning to be reduced.